What is double question mark in C#?

 The ?? operator is called the null-coalescing operator.

It returns the left-hand operand if it is not null; otherwise it returns the right operand

        int? x = null;

        // y = x, unless x is null, in which case y = -1.

        int y = x ?? -1

        // In here y = -1

        x = 1;

        y = x ?? -1

        // In here y = 1

Reference:

http://msdn.microsoft.com/en-us/library/ms173224.aspx